A technique using near-field light has been proposed as a method for reading and writing data from/on an optical disc medium with even higher density.
An optical system including a condenser lens and a solid immersion lens (which will be abbreviated herein as an “SIL”) in combination has recently attracted a lot of attention as a condensing means that produces near-field light. By using such a combination, a numerical aperture (NA) that is higher than that of a condenser lens can be achieved. Generally speaking, the higher the NA of an optical system, the smaller the spot size and the data can be stored with high density.
When a read/write operation is performed on an optical disc medium using an SIL, the gap between the SIL and the surface of the optical disc medium should be very narrow. In an optical system for use in DVDs (digital versatile discs), for example, there is a gap of approximately 1 mm between the objective lens and the surface of the optical disc medium. When an SIL is used, however, the gap between the emission plane of the SIL and the surface of the optical disc medium should be approximately 100 nm or less. If the gap between the SIL and the surface of the optical disc medium varied significantly, then the near-field light could fail to reach the surface of the optical disc medium or the SIL might collide against the optical disc medium. That is why a control needs to be performed to keep the gap between the SIL and the surface of the optical disc medium as constant as possible.
To get such a control done, a method called “gap servo” has been proposed in Patent Document No. 1, for example. According to that method, the intensity of a particular polarization component of the near-field light that has been reflected from the optical disc medium is detected and the positions of a condenser lens and an SIL in the optical axis direction are adjusted actively by an actuator to keep the intensity constant, thereby controlling the interval (or the gap) between the SIL and the optical disc medium.
In a normal recorder/player, the gap between the SIL and the surface of the disc should start to be controlled at a point in time after the recorder/player that has been turned ON has been loaded with the disc and before information is read or written from/on it. In conventional optical disc media such as DVDs, this operation corresponds to the operation of starting a focus control.
In conventional optical disc media, it is natural that the disc needs to be rotated when the focus control is started. This is because if the disc stayed put, a particular portion of the storage layer of the disc would be irradiated with, and locally heated by, a condensed laser beam continuously. In that case, the information stored in that particular portion might be erased or the storage layer or any other thin layer could deteriorate.
Nevertheless, once the gap has been controlled, the gap between the SIL and the surface of the optical disc medium should be as narrow as 100 nm or less. If the SIL has once been moved sufficiently away from, or has once contacted with, the surface of the optical disc medium, an extremely sophisticated technique would be required to provide a gap of 100 nm or less again between them. This is because if the movement of the SIL had even a small degree of overshoot or undershoot when it starts to be moved, the SIL might collide against the surface of the optical disc medium.
That is why before the gap control operation is started, various disturbance factors should be eliminated as much as possible. Once a disc has started rotating, the surface position of the disc in the optical axis direction will vary with time due to the disc flutter (or out-of-plane vibrations) or the unevenness of the surface. As such a variation constitutes a disturbance for the control operation, the control operation could be started with the disc kept stopped. Patent Document No. 2 discloses a method for controlling the gap between the SIL and the disc surface with the disc stopped. According to such a method, the gap is controlled by detecting electrostatic capacitance between the SIL and the disc surface, and therefore, there is no need to irradiate the disc surface with laser light for control purposes.                Patent Document No. 1: Pamphlet of PCT International Application Publication No. 03/021583        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2001-23284        